Brazil. I'm actually not against a civ that is completely flavorless for the first half of the game, but the fact that's mostly just culture\tourism advantages... yawn. Note that I don't find tourism boring, just that tourism
bonuses are. France is more fun though.
Korea has tech advantages. powerful, boring, and a DLC cash grab ("buy this civ so you can win on a higher difficulty") all at the same time.
I hate tech advantages.
Babylon does have early flavour (defense). Technically this gives them a unique position, and I do like build civs, but still... yawn. Doesn't help that it has tech advantages. Find a more interesting and non-tech UA and I'd probably like those walls.
Mongols need some sort of CS conquest bonus. Dare I say, a GP? They can even lower the attack bonus if they did that. As is... I mean the Keshiks and Khans can be fun, but I don't know, never really enjoyed a game with 'em.
Russia, maybe I'd change my mind if I played as them again...
Spain, of course the main problem is "luck-based", but "boring" isn't far behind since, unless NWs are just the right distance away, they're either UP or OP and when they're OP it's not in a "yay, let's see just how amazing an empire I can make" sort of way like Morocco can be.
Not boring but mentioning them:
Venice is fun but probably OP. I've only barely won on Emporer with cooking with anyone besides Venice, but Venice in archipelago and no further cooking yielded an admittedly close Deity win. Yeah, I know, they handle high difficulties well, but still. I would consider cutting their extra trade routes by half.
I like America. That sight bonus is all kinds of fun from my POV. Just wanted to throw that out there. Still, I understand the other side of this argument
Same with India. I mean, from an historic standpoint their UA is dumb to be sure, but in gameplay terms I like it fine.